A lumen is a measurement of the brightness of a bulb, LED, torch or any other light emitting device. Use this tag to tag questions about the the use of lumens on light emitting items such as head torches, etc.
There is an excellent answer here detailing what a lumen is and how it is calculated. In short:
..Lumens is the SI unit for luminous flux, which is essentially the total amount of light in the visible spectrum emitted by a source in all directions. Lumens does not depend on distance from the light source, because it must be measured in all directions. A related unit, lux, measures light at a point, and is dependent on distance. 1 lux = 1 lm/m2. If you were to measure the lux at every point on an imaginary sphere around a light source, add them up, and multiply by the surface area of that sphere, you would get the total lumens output of the device. No matter what the diameter of the sphere, you would get the same number of lumens. As you increase the diameter, the lux measured at each point decreases, but the surface area and number of points you must measure at increase.